[gambiting]

Well, ask in countries which actually have that as law - Poland introduced it few years ago and the jury is still out on whether it made roads safer or not. I'd argue it didn't - mostly because as a driver your brain starts to associate lights = moving car. Which means that you start to ignore everything that doesn't have lights - bicycles, cars where someone forgot to turn the lights on. There were studies done on this and it's very inconclusive that it improves safety at all.

[icc97]

Given it's a safety feature the main reason not to do it would be if somehow it made things worse. Even if the effect is almost negligible you're still going to save a couple of lives per year.

Once LED lights are more standard and there's less issue of replacing bulbs or wasting electricity it seems a no brainer.

[gambiting]

Well, that's the whole separate point in this discussion - normal headlamp bulbs are 60W - so you are using 120W of power to run two of them. How many cars are on the road for a country the size of Poland at a busy time of the day? 100k cars? That's 12 megawatts of power being used every day just to run some lights - and that's not counting rear lights at all. Generating 12 megawatts of power using thousands of ICEs is a massive waste of fuel, and it's probably incredibly hard to prove that conclusively, but I wouldn't be surprised if burning that fuel caused more deaths overall than having the lights on saves. I don't know if we can find any studies that would say either way though. Automotive LEDs are still within 10-15W range each, so while better than halogens they are not free.

And finally, I think there was a Polish study that said that yeah, while with the lights on you are more likely to notice a car, you are less likely to notice a cyclist - and hitting the latter has much worse consequences than hitting the former. So it might have made things worse.